1. Field of the Invention
The present invention relates to driving LED (Light-Emitting Diode) lamps and, more specifically, to dimming the LED lamps.
2. Description of the Related Arts
LEDs are being adopted in a wide variety of electronics applications, for example, architectural lighting, automotive head and tail lights, backlights for liquid crystal display devices, flashlights, etc. Compared to conventional lighting sources such as incandescent lamps and fluorescent lamps, LEDs have significant advantages, including high efficiency, good directionality, color stability, high reliability, long life time, small size, and environmental safety.
The use of LEDs in lighting applications is expected to expand, as they provide significant advantages over incandescent lamps (light bulbs) in power efficiency (lumens per watt) and spectral quality. Furthermore, LED lamps represent lower environmental impact compared to fluorescent lighting systems (fluorescent ballast combined with fluorescent lamp) that may cause mercury contamination as a result of fluorescent lamp disposal.
However, conventional LED lamps cannot be direct replacements of incandescent lamps and dimmable fluorescent systems without modifications to current wiring and component infrastructure that have been built around incandescent light bulbs. This is because conventional incandescent lamps are voltage driven devices, while LEDs are current driven devices, requiring different techniques for controlling the intensity of their respective light outputs.
FIG. 1 illustrates a typical dimmer wiring configuration in conventional residential and commercial lighting applications. Predominantly, incandescent lamps operate off of alternating current (AC) systems. Specifically, a dimmer switch 10 is placed in series with an input voltage source 15 and the incandescent lamp 20. The dimmer switch 10 receives a dimming input signal 25, which sets the desired light output intensity of incandescent lamp 20. Control of light intensity of the incandescent lamp 20 is achieved by adjusting the RMS voltage value of the lamp input voltage (V-RMS) 30 that is applied to incandescent lamp 20. Dimming input signal 25 can either be provided manually (via a knob or slider switch) or via an automated lighting control system.
Many dimmer switches adjust the V-RMS by controlling the phase angle of the AC-input power that is applied to the incandescent lamp to dim the incandescent lamp. FIGS. 2A, 2B, and 2C illustrate typical lamp input voltage waveforms which are output to incandescent lamp 20. FIG. 2A illustrates a typical lamp input voltage waveform 30 when no dimming switch 10 is present, or when the dimmer switch 10 is set to maximum light intensity and the voltage signal from the input voltage source 15 is unaffected by the dimmer switch 10. FIG. 2B illustrates lamp input voltage 30 with a dimming effect based on leading edge phase angle modulation (i.e. a leading edge dimmer). In a leading edge dimmer, the dimmer switch 10 eliminates a section 32 having a period Td_off of the lamp input voltage 30 after the zero-crossings of the AC half-cycles and before the peaks. The input voltage 30 is unchanged during the period Td_on. As the dimming input signal 25 increases the desired dimming effect, the period Td_off of the eliminated section 32 increases, the period Td_on decreases, and the output light intensity decreases. For minimum dimming (maximum light intensity), the period Td_off of the eliminated section 32 becomes very small or zero.
FIG. 2C illustrates lamp input voltage 30 with a dimming effect based on trailing edge phase angle modulation (i.e. a trailing edge dimmer). Trailing edge dimmer switches operate by removing trailing portions 34 of AC voltage half-cycles, after peaks and before zero-crossings. The input voltage 30 is unchanged during the period Td_on. Again, as the dimming input signal 25 increases the desired dimming effect, the period Td_off of the removed sections 34 increases, the period Td_on decreases, and the light intensity decreases. For minimum dimming (maximum light intensity) the period Td_off of the eliminated section 34 becomes very small or zero.
Controlling the phase angle is a very effective and simple way to adjust the RMS-voltage supplied to the incandescent bulb and provide dimming capabilities. However, conventional dimmer switches that control the phase angle of the input voltage are not compatible with conventional LED lamps, since LEDs, and thus LED lamps, are current driven devices.
One solution to this compatibility problem uses an LED driver that senses the lamp input voltage 30 to determine the operating duty cycle of the dimmer switch 10 and reduces the regulated forward current through an LED lamp as the operating duty cycle of the dimmer switch 10 is lowered. However, the control methods employed in these conventional solutions are compatible with only a single type of dimming switch, e.g., leading edge or trailing edge. If an LED lamp designed for use with a leading edge dimmer switch is connected to a lamp input voltage 30 using a trailing edge dimmer switch or vice versa, the LED lamp will likely malfunction and/or fail.